The present invention relates to adhesive compositions for plastics. More particularly, it relates to heat curable adhesive compositions for use with poly(butylene terephthalate) substrates.
Thermoplastic resins based on poly(1,4-butylene terephthalate) have become well known in industry. They are used in widely diverse applications where their high tensile and impact strengths make them extremely valuable. In many of these applications it is necessary to bond a sheet, film or molded article of the resin to itself or to other materials.
Presently known adhesives generally fail to provide satisfactory bond strength in cementing poly(1,4-butylene terephthalate) to itself or other materials. For instance, adhesives based on polyvinyl acetate have not been found useful because they provide a low bond strength. Similarly, the commonly used epoxy adhesives, such as those sold in home repair kits which contain the diglycidyl ether of bisphenol-A and a hardener, such as ethylene triamine, have proved useful only for applications where a bond strength of about 500 psi is satisfactory.
Recently, there have been advances in the polymer arts wherein new liquid compositions are now available which may be cured by heat or light to form solid materials. Examples of such compositions are described in commonly assigned Crivello, U.S. Pat. Nos. 4,026,705; 4,058,401 and 4,173,551. Of particular interest is U.S. Pat. No. 4,173,551, relating to thermocurable compositions which include a cationically polymerizable organic material, an aromatic halonium salt thermoinitiator, and a cocatalyst selected from the group consisting of a copper salt, an organic acid, or mixtures thereof and which may be used as adhesives, as well as for many other applications.
Still more recently, in U.S. application Ser. No. 399,041, filed Jul. 16, 1982, now abandoned by L. T-K. Nyeu and assigned to the same assignee as the present invention, epoxy adhesives for use with poly(butylene terephthalate) resin substrates are described which exhibit improved bond strengths of more than 1000 psi. The adhesive compositions described therein are comprised of an epoxidic prepolymer, a diaryliodonium complex salt thermal initiator dissolved in a compatibilizing organic liquid, a copper carboxylate salt, and optionally, finely divided silica filler.
It has been discovered that adhesive compositions for poly(butylene terephthalate) described by Nyeu can be improved to exhibit even greater adhesive bond strengths by the addition of a solvent for poly(butylene terephthalate) which is also reactive with epoxy resins.